The growing demand for energy throughout the world has placed great emphasis on the exploration of new sources of energy. Harvesting energy directly from sunlight using photovoltaic cells is recognized as an important solution to the growing energy crisis and environmental pollution.
Bulk heterojunction (BHJ) polymer solar cells, comprising interpenetrating networks of a donor polymer semi-conductor and a fullerene derivative acceptor such as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), have attracted a great deal of attention by virtue of their easy solution processability, mechanical flexibility, and the low-cost large-area manufacturing.
Materials innovation is one of the major forces currently driving the performance of polymer solar cells (PSCs). The efficiency of a PSC is given by η=VOC×JSC×FF, where VOC is the open circuit voltage, JSC is the short circuit current, and FF is the fill factor. Improvement of any of these three factors yields a higher efficiency. The key focus areas of polymer design include engineering the bandgap and energy levels to achieve high JSC and VOC, enhancing planarity to attain high carrier mobility, and materials processability and stability.
Research efforts focus on improving the power conversion efficiency (PCE) of devices. PCE is a measure of how much power can be generated from a device, and thus directly affects the cost of the device. Current organic photovoltaic materials and devices typically exhibit a PCE of about 5-8%, which is still lower than the general goal of 10% PCE which is often sought for devices that are to be mass produced. As well, efforts have been made to improve processability of materials used in device fabrication.
All of the aforementioned design focus areas are inter-related. In an ideal case, all factors should be optimized in a single polymer, but this remains a significant challenge. Organic photovoltaic (OPV) devices based on an ideal material are predicted to have a power conversion efficiency (PCE)>10% based on theoretical models, provided a suitable low bandgap donor material is available.
As a result, polymers used for organic photovoltaic devices application are typically designed with the tendency to form ordered structures in films to facilitate charge transporting within the devices. In order to form an ordered structure in thin films, the polymer chains should be as planar as possible.
Solution processing is one advantage of organic electronics, especially for OPV, which allows the OPV cells/modules to be manufactured through high throughput low cost roll-to-roll processing. Due to the high degree of ordered packing, the current light harvesting materials used in organic photovoltaic devices normally require solvents with very a high boiling point, for example chlorobenzene or dichlorobenzene, for ink formulation and device fabrication. These solvents tend to be toxic as well as environmentally unsafe. Thus, to date, very few polymers can be printed, either due to poor solubility or the requirement for particular solvents, many of which are considered environmentally hazardous.